1. Field of the Invention
The present invention is broadly concerned with processes and apparatuses for continuously harvesting micro-and nano-particles from organic solution-laden supercritical fluids. More particularly, the invention pertains to separators or filters comprising porous membranes preferably formed of TiO.sub.2 supported on porous metal substrates such as porous, sintered stainless steel. A feed stream comprising the desired particles, a supercritical antisolvent for the particles (preferably CO.sub.2) and a solvent for the particles is contacted with the membrane layer of the filter under near-critical or supercritical conditions for the mixture of antisolvent and solvent. The desired particles are retained by the filter while the solvent and most of the antisolvent pass through the filter. In one embodiment, the processes and apparatuses are combined with the Precipitation with Compressed Antisolvents (PCA) processes. In another embodiment, a plurality of filters is utilized in parallel, thus providing continuous harvesting of the desired particles.
2. Description of the Prior Art
For pharmaceutical applications, CO.sub.2 is an ideal processing medium. Because of its relatively mild critical temperature (31.1.degree. C.), it is possible to exploit the advantages of near-critical operation at temperatures lower than 35.degree. C. Furthermore, CO.sub.2 is non-toxic, non-flammable, relatively inexpensive, recyclable, and "generally regarded as safe" by the FDA and pharmaceutical industry. Even though the critical pressure (73.8 bar or 1070 psi) of CO.sub.2 is relatively high, such operating pressures and equipment are fairly routine in large-scale separation processes involving supercritical CO.sub.2, such as the decaffeination of coffee beans and the extraction of hops.
Carbon dioxide is a non-polar solvent. As such, carbon dioxide is essentially a nonsolvent for many lipophilic and hydrophilic compounds (which covers most pharmaceutical compounds). Supercritical CO.sub.2 has been exploited both as a solvent and as a nonsolvent or antisolvent in pharmaceutical applications. The ability to rapidly vary the solvent strength, and thereby the rate of supersaturation and nucleation of dissolved compounds, is a unique aspect of supercritical technology for particle formation.
The relatively low solubilities of pharmaceutical compounds in unmodified carbon dioxide are exploited in the CO.sub.2 -based antisolvent processes wherein the solute of interest (typically a drug, polymer or both) is dissolved in a conventional solvent to form a solution. The preferred ternary phase behavior is such that the solute is virtually insoluble in dense carbon dioxide while the solvent is completely miscible with dense carbon dioxide at the precipitation temperature and pressure.
The solute is recrystallized from solution in one of two ways. In the first method, a batch of the solution is expanded several-fold by mixing with dense carbon dioxide in a vessel. Because the carbon dioxide-expanded solvent has a lower solvent strength than the pure solvent, the mixture becomes supersaturated forcing the solute to precipitate or crystallize as micro-particles. This process is generally referred to as Gas Antisolvent (GAS) precipitation (Gallagher et al., 1989 Gas Antisolvent Recrystallization: New Process to Recrystallize Compounds in Soluble and Supercritical Fluids. Am. Chem. Symp. Ser., No. 406; U.S. Pat. No. 5,360,487 to Krukonis et al.; U.S. Pat. No. 5,389,263 to Gallagher et al.).
The second method involves spraying the solution through a nozzle into compressed carbon dioxide as fine droplets. This process is referred to as Precipitation with Compressed Antisolvents (PCA) (Dixon et al., AIChE J., 39:127-39(1993)) and employs either liquid or supercritical carbon dioxide as the antisolvent. When using a supercritical antisolvent, the spray process is referred to as Supercritical Antisolvent (SAS) Process (Yeo, Biolech. Bioeng., 41:341-46 (1993)) or Aerosol Spray Extraction System (ASES) process (Muller et al., Verfahren zur Herstellung einer mindestens einen Wirkstoffund einen Trager umfassenden Zubereitung, German Patent Appl. No. DE 3744329 A1 1989.).
The foregoing references demonstrate that techniques using carbon dioxide as a nonsolvent can produce drug particles in a narrow size distribution using fewer organic solvents. Because the spray-processes (PCA, SAS and ASES) permit faster depletion of the solvent (and hence a greater production rate of particles) relative to the GAS process, they have received more attention in recent years.
The particles formed in the recrystallizer during a PCA process have to be recovered without significantly decreasing the pressure or temperature. Otherwise, the solvent would separate from the CO.sub.2 phase and re-dissolve the particles. In laboratory proof-of-concept studies involving particle micronization, only microgram to a few milligram quantities of particles are formed by spraying for a few minutes. These particles are collected after spraying is stopped and the system is flushed with dense carbon dioxide for a sufficient period of time to reduce the solvent concentration to negligible proportions. The system pressure is then reduced to ambient pressure, and the particles are collected from the crystallizer. Clearly, this method of harvesting particles is not suited for continuous production of particles. Continuous particle production and harvesting is necessary in order to produce particles on the order of g/hr or on a larger commercial scale of kg/hr. Therefore, a process in which the solvent is continuously separated from the CO.sub.2 /solvent/particles mixture is desirable.
Cyclone separators have been employed to separate the particles from a stream containing the particles and solvent-loaded CO.sub.2. In this method, the particles generated in the crystallization chamber are continuously separated in a downstream high-pressure cyclone separator. The effluent stream from the cyclone separator is led to a flash drum operated at decreased pressures where the solvent and the CO.sub.2 phases are separated and recycled. Cyclone separators work best for separating particles 5 .mu.m or greater and are generally not effective for separating submicron or nanoparticles.
Electrostatic precipitation is another viable method to harvest nanoparticles. However, currently available electrostatic precipitators are rated up to only 10 bar (or about 738 psi). Hence, custom design and fabrication of an electrostatic precipitator for operation at supercritical conditions is needed. Another disadvantage of electrostatic precipitation is that the static charge tends to cause particle agglomeration.